I disagree. I found thatches to be very common but maybe that's because I concentrated on my home/greenhouse-like place than expanding on my raft.
When I was playing on my second save file, I decided to make a huge greenhouse-like place for my coconut trees and food and drink sources. If you make enough tree cropplots (I made about seven), you'll have enough thatch and wood to continue your raft building. But if you don't have enough or any cropplots, you can make a lot of item nets (I made about six) to help you farm those thatches you need. It'll take a while to gather the thatches you need but it'll help if you build something else while waiting. In time, you'll gather a lot of thatches this way.